feat(admin): add drag-and-drop group sort order

- Add `sort_order` field to groups table with migration
- Add `PUT /api/v1/admin/groups/sort-order` API for batch update
- Implement drag-and-drop UI using vue-draggable-plus
- All queries now order groups by sort_order
- Add i18n support (en/zh) for sort-related UI text
- Update test stubs to satisfy new interface methods

backend/internal/service/admin_service.go

@@ -36,6 +36,7 @@ type AdminService interface {
 	UpdateGroup(ctx context.Context, id int64, input *UpdateGroupInput) (*Group, error)
 	DeleteGroup(ctx context.Context, id int64) error
 	GetGroupAPIKeys(ctx context.Context, groupID int64, page, pageSize int) ([]APIKey, int64, error)
+	UpdateGroupSortOrders(ctx context.Context, updates []GroupSortOrderUpdate) error
 
 	// Account management
 	ListAccounts(ctx context.Context, page, pageSize int, platform, accountType, status, search string) ([]Account, int64, error)
@@ -1015,6 +1016,10 @@ func (s *adminServiceImpl) GetGroupAPIKeys(ctx context.Context, groupID int64, p
 	return keys, result.Total, nil
 }
 
+func (s *adminServiceImpl) UpdateGroupSortOrders(ctx context.Context, updates []GroupSortOrderUpdate) error {
+	return s.groupRepo.UpdateSortOrders(ctx, updates)
+}
+
 // Account management implementations
 func (s *adminServiceImpl) ListAccounts(ctx context.Context, page, pageSize int, platform, accountType, status, search string) ([]Account, int64, error) {
 	params := pagination.PaginationParams{Page: page, PageSize: pageSize}

backend/internal/service/admin_service_delete_test.go

@@ -172,6 +172,10 @@ func (s *groupRepoStub) GetAccountIDsByGroupIDs(ctx context.Context, groupIDs []
 	panic("unexpected GetAccountIDsByGroupIDs call")
 }
 
+func (s *groupRepoStub) UpdateSortOrders(ctx context.Context, updates []GroupSortOrderUpdate) error {
+	return nil
+}
+
 type proxyRepoStub struct {
 	deleteErr    error
 	countErr     error

backend/internal/service/admin_service_group_test.go

@@ -116,6 +116,10 @@ func (s *groupRepoStubForAdmin) GetAccountIDsByGroupIDs(_ context.Context, _ []i
 	panic("unexpected GetAccountIDsByGroupIDs call")
 }
 
+func (s *groupRepoStubForAdmin) UpdateSortOrders(_ context.Context, _ []GroupSortOrderUpdate) error {
+	return nil
+}
+
 // TestAdminService_CreateGroup_WithImagePricing 测试创建分组时 ImagePrice 字段正确传递
 func TestAdminService_CreateGroup_WithImagePricing(t *testing.T) {
 	repo := &groupRepoStubForAdmin{}
@@ -395,6 +399,10 @@ func (s *groupRepoStubForFallbackCycle) GetAccountIDsByGroupIDs(_ context.Contex
 	panic("unexpected GetAccountIDsByGroupIDs call")
 }
 
+func (s *groupRepoStubForFallbackCycle) UpdateSortOrders(_ context.Context, _ []GroupSortOrderUpdate) error {
+	return nil
+}
+
 type groupRepoStubForInvalidRequestFallback struct {
 	groups  map[int64]*Group
 	created *Group
@@ -466,6 +474,10 @@ func (s *groupRepoStubForInvalidRequestFallback) BindAccountsToGroup(_ context.C
 	panic("unexpected BindAccountsToGroup call")
 }
 
+func (s *groupRepoStubForInvalidRequestFallback) UpdateSortOrders(_ context.Context, _ []GroupSortOrderUpdate) error {
+	return nil
+}
+
 func TestAdminService_CreateGroup_InvalidRequestFallbackRejectsUnsupportedPlatform(t *testing.T) {
 	fallbackID := int64(10)
 	repo := &groupRepoStubForInvalidRequestFallback{

backend/internal/service/gateway_multiplatform_test.go

@@ -290,6 +290,10 @@ func (m *mockGroupRepoForGateway) GetAccountIDsByGroupIDs(ctx context.Context, g
 	return nil, nil
 }
 
+func (m *mockGroupRepoForGateway) UpdateSortOrders(ctx context.Context, updates []GroupSortOrderUpdate) error {
+	return nil
+}
+
 func ptr[T any](v T) *T {
 	return &v
 }

backend/internal/service/gemini_multiplatform_test.go

@@ -226,6 +226,10 @@ func (m *mockGroupRepoForGemini) GetAccountIDsByGroupIDs(ctx context.Context, gr
 	return nil, nil
 }
 
+func (m *mockGroupRepoForGemini) UpdateSortOrders(ctx context.Context, updates []GroupSortOrderUpdate) error {
+	return nil
+}
+
 var _ GroupRepository = (*mockGroupRepoForGemini)(nil)
 
 // mockGatewayCacheForGemini Gemini 测试用的 cache mock

backend/internal/service/group.go

@@ -45,6 +45,9 @@ type Group struct {
 	// 可选值: claude, gemini_text, gemini_image
 	SupportedModelScopes []string
 
+	// 分组排序
+	SortOrder int
+
 	CreatedAt time.Time
 	UpdatedAt time.Time
 

backend/internal/service/group_service.go

@@ -33,6 +33,14 @@ type GroupRepository interface {
 	GetAccountIDsByGroupIDs(ctx context.Context, groupIDs []int64) ([]int64, error)
 	// BindAccountsToGroup 将多个账号绑定到指定分组
 	BindAccountsToGroup(ctx context.Context, groupID int64, accountIDs []int64) error
+	// UpdateSortOrders 批量更新分组排序
+	UpdateSortOrders(ctx context.Context, updates []GroupSortOrderUpdate) error
+}
+
+// GroupSortOrderUpdate 分组排序更新
+type GroupSortOrderUpdate struct {
+	ID        int64 `json:"id"`
+	SortOrder int   `json:"sort_order"`
 }
 
 // CreateGroupRequest 创建分组请求